UPS carrying out ACs

[m sleem]

Ok, that would be a strange practise. I would think it is not a code violation but i need your opinion in this case.
There is a server room and the power supply for its ACs is not stable. The owner already supplied an ups with sufficient capacity. My question, do we have to consider the starting current of the ACs? means that the UPS has to be more than the 2.5*largest motor+other loads?

 

[iwire]

UPS systems will not like heavy motor loads, the in rush will be 6 to 8 times the full load current and might just make the UPS shut down.

Also many UPS systems do nothing to stabilize the voltage unless they are expensive Online / double-conversion units.

http://en.wikipedia.org/wiki/Uninterruptible_power_supply#Online_.2F_double-conversion

 

[brian john]

The UPS will go to bypass from the overload, depending on how oversized the UPS is. when the current is within operating range the UPs will switch back to Normal mode.

Larger UPS over load ratings are like 150% for less than a second and 110-115 percent for 10 seconds (This is from memory not a particular UPS literature)


Having said that in 33 years of working in data centers I have only seen a AC on UPS a few times and one time they put in a designated UPS for AC's only. But overloads are very common such as PDU's with stepdown transformers.

 

[eHunter]

There are several issues with A/C units or large motor loads supplied by a UPS.
Most, but not all UPS systems will go into bypass mode during high current/overload conditions such as motor starting.
This is a great feature until there is an A/C or motor startup that occurs when there is a utility failure and no available utility power to bypass to and the units fail to start or the UPS shutsdown on overload.
If the A/C units are single phase investigate if the manufacturer offers "hard start kits" that can help ease startups.
If there are more than one A/C unit, add a unique 2-5 minutes time delay to each unit to allow refrigerant pressure equalization and stagger the startup of each unit reducing the total current load during startup.

What is the voltage, phase, running current, starting current or LRA, size(BTU or KW) and quantity are the A/C unit(s)?
Are the A/C units integrated CRAC, package, ductless split units, CRAH or other?
What brand, model, phase, voltage and KW is the UPS?
Does this UPS have an integrated bypass switch?
Will this UPS be backed up by a standby generator?

 

[ceb58]

I would hope that you have a stand by generator to handle the room and A/C unit(s). In our communication buildings if it is running on generator and one of the 3 ton A/C units kicks on the UPS will go into by pass until the voltage stabilizes. The only thing we allow on the UPS circuits is the communication equipment.

 

[eHunter]

I would hope that you have a stand by generator to handle the room and A/C unit(s). In our communication buildings if it is running on generator and one of the 3 ton A/C units kicks on the UPS will go into by pass until the voltage stabilizes. The only thing we allow on the UPS circuits is the communication equipment.

That is exactly why I was enquiring.
I have also experienced finicky UPS systems that did not always like genset supplied power at all without pre-conditioning.

 

[zbang]

I've never seen any point to putting the HVAC on a UPS, since outside of exceptional cases, loosing it for maybe 30 seconds while the generator starts isn't a problem (or while some systems do a controlled shutdown). I can see putting some of the fans on UPS to keep the air moving, but not compressors or pumps. (Remember stuff like staggered startup of compressors/etc on the generator and all that.) It also depends on the actual heat load vs available cooling. If the HVAC must run continually just to keep things from melting, you may have other problems during an outage.

 

[GoldDigger]

I've never seen any point to putting the HVAC on a UPS, since outside of exceptional cases, loosing it for maybe 30 seconds while the generator starts isn't a problem (or while some systems do a controlled shutdown).

Unless your AC equipment requires a several minute bleed-down before restarting the compressor because it cannot start under normal head pressure....
Keeping then fans running during that wait may be enough though, as long as the start attempt, drawing LRA till the motor overload kicks in, does not overload the generator.